A frequency converter is an electrical apparatus that can be used for controlling a rotating electrical machine. For the control, the frequency converter can be synchronized with the machine such that the frequency converter knows the electrical properties of the machine. These electrical properties include the rotational direction and rotational speed of the controlled machine.
In a frequency converter, the part controlling the load is an inverter. The inverter can include output switches with which a desired voltage is switched to the load. Free-wheeling diodes are connected antiparallel to each output switch to enable a flow of currents in the load phases.
A frequency converter also can include an input bridge which converts an AC input voltage to a DC voltage from which the output switches for an alternating voltage to the load. A diode bridge is a known structure of the input bridge. Some frequency converters are equipped with a possibility to transfer electrical energy back to the supplying network. These line converters are basically similar to the inverters having active switch components and diodes connected antiparallel with the active components. As with the inverters controlling the load, the line converters can be synchronized with the rotating voltage source (e.g., the supplying network).
In some cases, the inverter is not synchronized with a rotating voltage source. This may be the case when, for example, the line converter is connected to the supplying network or when the network returns after a failure in the supplying network. Similarly, when the inverter controlling a rotating machine loses the synchronization or is connected to a rotating machine, the inverter should obtain knowledge of the electrical properties of the rotating load before it may start the modulation of the switches. If the rotating load is a permanent magnet synchronous machine, the load acts as a generator and provides power to the intermediate circuit of the frequency converter via the diodes of the inverter bridge if the rotational speed of the machine is high enough to produce a voltage which is higher than the voltage of the supplying mains.
The synchronization to the rotating voltage source can be achieved by known methods, which include a zero-current control and a short circuit pulse method. In the zero-current control, a zero current is set to target for the current controller of the inverter. Once no current flows between the inverter and the rotating load, the voltage of the inverter corresponds to the voltage of the load and synchronization is obtained. With zero-current control, the current controller should be quite fast to avoid a large current pulse at the beginning of the method. The short circuit pulse method can be inaccurate when the method is based on few test pulses only.
When the rotational speed and thus the motion voltage are higher, currents can change faster. Especially a short circuit pulse test is disturbed once the machine is in a field weakening region when a motion voltage generated by the machine is at the same level as the voltage of a DC bus of the frequency converter. Due to a low level of the DC bus voltage, a current vector which occurs during the zero voltage does not fade away quickly when the inverter stops modulating.